The present invention relates to radar and, in particular, to moving-target indicator (MTI) radar employing signal changes due to target velocity to discriminate a target from background clutter.
As used herein, moving-target indicator radar is a general term which encompasses low PRF systems of the delay canceller or fast Fourier transform types and high PRF types sometimes known as pulse doppler. The unifying characteristic of MTI radar is the use of a difference in the signal returned from a target moving at normal target speeds as compared to background clutter or noise.
The doppler shift in the received frequency of a radar returned from a target moving relative to the radar system is given by fd=2v.sub.r /.lambda.
Where:
v.sub.r =relative velocity PA1 fd=doppler shift PA1 .lambda.=wavelength PA1 P.sub.t =transmitter power PA1 G.sub.t =transmit antenna gain PA1 A.sub.r =receive antenna effective capture area PA1 .sigma.=radar cross section PA1 R=range to target PA1 P.sub.t =transmitter power PA1 R=range to target PA1 G=antenna gain PA1 .sigma.=radar cross section
A stationary radar system having its field of view generally horizontal searching or tracking ground based or airborne moving objects obtains returns not only from the target object itself, but also from the surface of the earth, objects both natural and man-made extending above the earth and from particles, principally water and ice, in the air. Early radar systems employing simple cancellers stored the radar return from one pulse and subtracted corresponding portions of the radar return received from a second pulse. The theory being applied is that the radar returned from stationary objects is the same in successive pulses and, therefore, the subtraction substantially eliminates such stationary objects leaving only moving objects to be displayed. More advanced systems used the returns from more than two pulses and employed various types of transforms to enhance the detectability of moving targets in the presence of clutter. Such systems permitted detection of the moving targets in the presence of background clutter having many times the return signal strength as the target itself. The ratio of clutter signal strength to detectable target signal strength is known as "subclutter visibility". Moving target indicator systems have given subclutter visibilities in the range of 20 to 30 decibels.
Even in stationary radar systems, the clutter itself is not necessarily stationary and, therefore, is not perfectly cancelled by canceller type MTI systems. That is, in addition to the moving target and stationary ground returns, the return signal contains components from wind-moved vegetation such as trees, etc., and from wind-driven moisture such as rain storms.
Doppler frequency signal processors overcome many of these difficulties and also permit the use of MTI systems in moving platforms such as aircraft wherein a doppler component is produced in the clutter return due to the motion of the radar-carrying platform. In such systems, the radar return is frequency filtered into a number of parallel channels. The principal ground return is often employed in a clutter tracker to control a local oscillator which positions the frequency of the stationary ground return at a predetermined value so that moving target and moving clutter returns will fall within predictable frequency bands.
Radar systems normally determine the presence or absence of a target according to whether the returned signal strength exceeds or fails to exceed a threshold level. The setting of such a threshold level is always a compromise in radar systems. If the threshold is set too low, noise present in the signal will exceed the threshold even without the presence of a target. Such false indications of target are called false alarms. Conversely, if the threshold is set too high, even signals containing returns from real targets may not exceed the threshold and, therefore, the probability of detecting the target is degraded.
It is thus seen that false alarms and probability of detection are mutually antagonistic.
The power returned from a target in a monostatic radar (transmitter and receiver in the same location) is given by ##EQU1## Where: P.sub.r =power received
When the same antenna is employed for both transmitting and receiving, the receipt of power is given by the following: ##EQU2## Where: P.sub.r =received power
Except for radar cross section .sigma. and range, the right-hand side of the last equation can be considered a constant. As is well known, the radar cross section of a target is strongly variable with target geometry and orientation. The same is true for background and foreground clutter returns. This, of course, complicates the setting of threshold level for target detection.
The inverse fourth power range relationship also complicates the setting of a detection threshold level. That is, as a packet of radar energy travels outward from the antenna, the returns received back at the antenna vary as a function of the inverse fourth power of range.
In electronic counter-countermeasure systems, wherein intentional jamming is employed to degrade the detection probability, the false alarm rate can increase to an intolerable level such that the radar system becomes unusable. In operator controlled systems, where an operator monitors a radar display (essentially acting as a human threshold), in the presence of jamming, the operator turns down the gain of the radar system to a level which reduces the false alarm rate to a level he can tolerate. In automatic systems, the average noise level may be used to provide automatic gain control to maintain the false alarm rate at a constant level. Such a radar system is known as a constant false alarm receiver (CFAR). Since automatic CFAR reacts faster than a human, it is superior to the attempt of an operator to keep the false alarm rate constant.
The same result can be obtained by automatically raising the threshold level at which target detection is recognized. It should be understood that either automatic method of maintaining a constant false alarm rate reduces the probability of detection. Furthermore, the gain or threshold control may not be controlled rapidly enough to account for the inverse fourth power of range factor in the return signal.